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Revista Brasileira de Zootecnia

versão impressa ISSN 1516-3598versão On-line ISSN 1806-9290

R. Bras. Zootec. vol.48  Viçosa  2019  Epub 28-Out-2019

http://dx.doi.org/10.1590/rbz4820180232 

Non-ruminants

Full-length research article

Growth performance and intestinal health of broilers fed a standard or low-protein diet with the addition of a protease

Kátia Maria Cardinal1  * 
http://orcid.org/0000-0002-8419-509X

Mariana Lemos de Moraes2 
http://orcid.org/0000-0003-3497-8106

Ines Andretta1 
http://orcid.org/0000-0002-6205-4229

Graciele Dalise Schirmann1 
http://orcid.org/0000-0002-2586-8581

Bruna Luiza Belote3 
http://orcid.org/0000-0001-8412-5107

Miguel Alejandro Barrios2 
http://orcid.org/0000-0001-5725-8641

Elizabeth Santin3 

Andréa Machado Leal Ribeiro1 
http://orcid.org/0000-0003-0243-7946

1Universidade Federal do Rio Grande do Sul, Departamento de Zootecnia, Porto Alegre, RS, Brasil.

2Jefo Nutrition Inc., Saint-Hyacinthe, Quebec, Canada.

3Universidade Federal do Paraná, Departamento de Medicina Veterinária, Laboratório de Microbiologia e Ornitopatologia, Curitiba, PR, Brasil.


ABSTRACT

We evaluated the effects of a protease supplementation on the growth performance and intestinal health of broilers. Cobb chicks (392; 1-42 d) were divided into four treatments (seven replicates of 14 birds each). There were two feed formulations: a standard diet (SD) and a low crude protein and digestible amino acids diet (Low CP&AA). The two diets were either supplemented (+P) or not (−P) with a protease (Jefo Protease; 1.25 g kg−1). Performance was evaluated by feeding phases (1-7, 8-21, 21-35, and 35-42 d). On day 28, ileum samples were analyzed by a morphometric index for histological alterations (I See Inside Scoring System – ISI). Broilers fed the Low CP&AA had a poor feed conversion ratio (FCR); however, the addition of the protease to the Low CP&AA positively affected FCR and body weight gain and promoted a performance similar to the group fed SD−P. Birds fed diets supplemented with the protease presented the best ISI morphological index, mainly as a result of the low number of alterations regarding the lamina propria, epithelial thickness, and enterocyte proliferation. It is possible to conclude that the enzyme improves feed conversion and lamina propria, epithelial thickness, and proliferation of enterocytes index of broiler chickens when added to a standard diet or with a low crude protein and digestible amino acids diet.

Key words: aminoacid; animal nutrition; digestive physiology; enzyme; feed additive

Introduction

The constant improvement in performance indexes observed in the poultry production depends on the proper formulation of diets to provide all the necessary nutrients for the chickens. To achieve this goal, increasingly specific diets are formulated and applied, considering growing phase, genetic line, and climate. There is interest in the use of low-crude protein diets to combine productive indexes with economic and environmental advantages. However, when there is reduction of the crude protein (CP) of the diet, some factors must be evaluated, such as the reduction of essential amino acids. The reduction of dietary CP content by 2.5% compared with the recommended value negatively affects broiler growth performance and blood parameters, but when threonine is included in low-CP diet, weight gain and feed intake (FI) are improved, as well as serum lipid profile ( Sigolo et al., 2017 ). Poultry diets are frequently formulated on a least cost; consequently, feed contains protein from different ingredients and may vary in amino acid bioavailability ( Ammerman et al., 1995 ; Kong and Adeola, 2014 ). Exogenous protease supplementation may improve the utilization of amino acids of the diet ( Olukosi et al., 2007 ; Freitas et al., 2011 ), and nutritionists can formulate diets with lower levels of dietary protein while maintaining growth performance, promoting sustainability of poultry production in general ( Leinonen and Williams, 2015 ).

The secondary effects of exogenous proteases have been the focus of recent inquiries. Cowieson et al. (2018) concluded that exogenous protease has beneficial effects in combination with ascorbic acid on broiler feed conversion, amino acid digestibility, and intestinal integrity. The mechanisms by which a protease contributes to positive gut health are not entirely clear and may be a combination of different factors ( Cowieson and Roos, 2016 ). Previous researchers have demonstrated that exogenous proteases may contribute to a shift of substrates available in the intestine for bacterial growth ( Bedford and Partridge, 2001 ; Malo et al., 2010 ). Morphological changes in the intestine are associated with the presence of nutrients in the lumen of the small intestine. The excess of undigested protein relates to putrefaction in the distal portion of the intestine, which can cause proliferation of pathogenic bacteria and increase the incidence of necrotic enteritis ( Williams, 2005 ).

The gut undergoes major morphological and functional modifications after hatching, such as increase in intestine length, villi height and density, and consequently, number of goblet cells, enterocytes, and enteroendocrine cells ( Boeli et al., 2002 ). Transporters present in the membrane of the epithelial cells of the mucosa are responsible for nutrient absorption ( Boeli et al., 2002 ; Pelicano et al., 2003 ; Khambualai et al., 2010 ). The presence of inflammation on intestinal mucosa affects broiler weight gain and feed conversion ( Kraieski, 2017 ); therefore, the integrity of the intestine is of fundamental importance, since it is the entry site of nutrients for broiler development.

The objective of this study was to evaluate the impacts of an exogenous protease supplemented to low-protein and low-amino acid diets on performance and intestinal health of broiler chickens.

Material and Methods

Research on animals was conducted according to the local institutional committee on animal use (case number 33342). The present study was conducted in Porto Alegre, Rio Grande do Sul, Brazil (Latitude: −30.0277 S, Longitude: −51.2287 30°1'40" S, 51°13'43" W).

A total of 392 day-old male broilers (Cobb 500; ±٤٩.٥ g) were obtained from a commercial hatchery and housed for 42 days in an experimental facility with 28 floor pens of 1 m2 each, covered with wood shavings, and equipped with nipple drinkers and metal hanging feeders. Groups of 14 birds were randomly assigned to one of the 28 pens, and the variation of average body weight to each experimental unit (pen) did not exceed 3%. Broilers were kept in thermal comfort throughout the trial. Body weight (BW) and FI were measured per feeding phase (1-7, 8-21, 21-35, and 35-42 d) to calculate body weight gain (BWG) and feed conversion ratio (FCR).

Diets based on corn and soybean meal were formulated for four phases (1-7, 7-21, 21-35, and 35-42 d; Table 1 ). Two different diets were formulated for each phase: a standard diet (SD), according to the nutritional recommendations of the Brazilian Tables for Poultry and Swine ( Rostagno et al., 2011 ) and a low-protein diet (Low CP&AA), with 6% reduction on CP and main digestible amino acids (lysine, methionine, threonine, and tryptophan). Each diet formulation was either supplemented (+P) or not (−P) with a protease from Streptomyces griseus (Jefo Protease, Jefo Nutrition Inc., Saint-Hyacinthe, Canada; 1.25 g kg−1), totaling four treatments in a 2×2 factorial arrangement. Diets and water were offered ad libitum .

Table 1 Composition of diets (as fed basis) 

Item 1-7 days 7-21 days 21-35 days 35-42 days




Standard1 −6%2 Standard −6% Standard −6% Standard −6%
Ingredient (g kg−1)
Soybean meal 396 364 365 329 330 297 295 266
Corn 533 572 561 604 591 631 630 665
Soybean oil 27 20 34 27 43 37 43 37
L-lysine 2 2 3 3 2 2 2 2
DL-methionine 2 2 1 1 1 1 1 1
L-threonine 1 0 0 0 0 0 0 0
NaCl 5 5 4 4 4 4 4 4
Limestone 5 5 6 6 6 6 6 6
Phosphate 24 24 19 20 16 16 13 13
Choline 0 0 0 0 0 0 0 0
Vit-min premix3 1 1 1 1 1 1 1 1
Nutritional composition4 (g kg−1)
Crude protein 224.0 212.5 212.0 199.3 198.0 186.1 185.2 174.8
ME (kcal kg−1) 2,960 2,960 3,050 3,050 3,150 3,150 3,200 3,200
Dig Lys 13.2 12.5 12.8 12.0 11.3 10.6 10.6 10.0
Dig Met 5.2 4.9 4.8 4.5 4.5 4.3 4.2 4.0
Dig Met+Cys 9.0 8.5 8.4 8.0 8.0 7.6 7.6 7.3
Dig Thr 8.6 8.1 7.9 7.4 7.4 6.9 6.9 6.5
Dig Trp 2.6 2.1 2.4 2.2 2.2 2.0 2.0 1.9
Dig Arg 14.3 13.4 13.4 12.4 12.4 11.5 11.5 10.7
Dig Val 9.8 9.4 9.3 8.8 8.8 8.3 8.2 7.8

1 Based on the Brazilian Tables for Poultry and Swine ( Rostagno et al., 2011 ).

2 6% reduction of crude protein and digestible amino acids.

3 Composition (content per kg of diet): 150,000 mg Mn; 100,000 mg Zn; 80,000 mg Fe; 15,000 mg Cu; 1,200 mg I; 700 mg Se; 23,200,000 IU vitamin A; 5,600,000 IU vitamin D; 52,000 mg vitamin K; 6,000 mg vitamin B1; 18,000 mg vitamin B2; 9,000 mg vitamin B6; 132,000 mg niacin; 44,000 mg panthotenic acid; 2,400 mg folic acid; 200,000 μg biotin; 40,000 μg vitamin B12.

4 Nutritional composition was analyzed.

At 28 d, a fragment of approximately 5 cm length of ileum end portion was collected from seven birds per treatment for intestinal health analysis. The segment was placed on a rectangular piece of cardboard, which was sectioned longitudinally, clipped, washed with saline solution, and immediately stored individually immersed in Davidson solution. The samples were sent to the laboratory to be analyzed macroscopic and microscopically by the ISI (“I See Inside”) methodology (patent INPI UFPR 10 2015 003601 9), which is based on a numeric score of alteration. In this methodology, an impact factor (IF) is defined for each alteration in macroscopic and microscopic analysis, according to the reduction of organ functional capacity, based on previous knowledge from the literature and background research. The IF ranges from 1 to 3, with 3 being the most impacting to organ function. The parameters evaluated by the ISI methodology were: lamina propria thickness, epithelial thickness, proliferation of enterocytes, epithelial plasma infiltration, mixed inflammatory infiltration in the lamina propria, goblet cell proliferation, congestion, necrosis (apical karyolysis), and presence of Eimeria oocysts .

In addition, the extent of each lesion (intensity) or the observed frequency compared to non-affected organ is evaluated in each organ/tissue with score (S) ranging from 0 to 3: score 0 (absence of lesion or frequency), score 1 (alteration up to 25% of the area or observed frequency), score 2 (alteration ranging from 25 to 50% of the area or observed frequency), and score 3 (alteration extends to more than 50% of the area or observed frequency). To obtain the final value of the ISI index, the IF of each alteration is multiplied by the respective score number, and the results of all alterations are summed according to the formula ISI = Σ(IF∗S). For example, the lamina propria thickness has IF = 2, and this number will be multiplied by the observed score (ranging from 1 to 3); if a score S = 3 (maximum score) is observed for lamina propria thickness in the villi, the ISI for this parameter in the villi will be ISI = (2∗3) = 6. The average of 20 villi observed in each bird will reach the final value for this parameter, and the sum of the average of all parameters already mentioned, will give the total ISI value for this specific bird (each bird is a replicate for statistical analysis).

Each pen was considered an experimental unit and there were seven replicates (pens) per treatment. Data were analyzed by ANOVA using the General Linear Model procedure considering dietary level of protein and supplementation with protease as main factors. In the presence of a significant effect (F), means were separated by Fisher LSD using a statistic software (Statistical Analysis System, version 9.4). For histological analysis, each bird was considered a replicate. At first, data normality was verified using Shapiro-Wilk normality test. Rates were compared using one-way analysis of variance (ANOVA) followed by Tukey’s test (P<0.05). The histological analysis was performed with Statistix 9 software for Windows. Variance analysis was performed considering the following statistical model:

Yijk = m + αi + βj + (αβ)ij + eijk,

in which Yijk is the result, m is the mean of all experimental units for the study variable, αi is the effect of protease supplementation, βj is the effect of protein level, (αβ)ij is the interaction between factors, and eijk is the error associated with observation.

Results

In the first and second periods of the trial (1-7 and 7-21 d), the enzyme supplementation did not affect performance (P>0.05), whereas Low CP&AA negatively affected BW, WG, and FCR ( Table 2 ; P<0.05). Protease supplementation showed the same positive effect from 21 d. In the total period of rearing (1-42 d), WG and FCR were positively influenced by protease, while Low CP&AA negatively influenced FCR (P<0.05). In the interaction ( Table 3 ), Low CP&AA−P showed the worst FCR from 7 to 21 and 21 to 35 d (P<0.05). Protease supplementation improved FCR (P<0.05), and the Low CP&AA–P presented the worst result of FCR in the total period (1-42 d).

Table 2 Growth performance of broilers fed diets containing different protein and digestible amino acid levels supplemented or not with protease 

Protein level Protease supplementation SEM


Standard diet1 6% reduction2 P3 −Protease +Protease P4
1-7 d
BW 7 d 169 168 0.678 168 169 0.880 0.93
FI 163 168 0.515 168 164 0.481 3.53
BWG 120 119 0.631 119 119 0.857 0.91
FCR 1.36 1.41 0.314 1.43 1.40 0.370 0.02
7-21 d
BW 21 d 861a 822b 0.032 839 844 0.746 9.03
FI 1020 1017 0.895 1009 1028 0.339 9.08
BWG 692a 653b 0.024 670 675 0.741 9.05
FCR 1.47a 1.55b 0.003 1.51 1.52 0.605 0.01
21-35 d
BW 35 d 2143 2069 0.073 2091 2122 0.442 16.9
FI 2094 2144 0.363 2158 2080 0.165 28.1
BWG 1282 1247 0.327 1252 1277 0.473 17.0
FCR 1.63a 1.72b 0.001 1.72b 1.62a <0.001 0.01
35-42 d
BW 42 d 2792 2716 0.163 2696b 2811a 0.040 28.2
FI 1102 1110 0.902 1065 1146 0.214 32.8
BWG 647 646 0.974 605b 688a 0.018 17.8
FCR 1.83 1.72 0.876 1.76 1.66 0.071 0.02
1-42 d
FI 4380 4441 0.511 4402 4418 0.859 46.4
BWG 2742 2667 0.162 2647b 2762a 0.039 28.2
FCR 1.59a 1.67b 0.001 1.66b 1.60a 0.002 0.01

BW - body weight; FI - feed intake; BWG - body weight gain; FCR - feed conversion ratio; SEM - standard error of the mean.

1 Based on the Brazilian Tables for Poultry and Swine ( Rostagno et al., 2011) .

2 6% reduction of crude protein and digestible amino acids.

3 P-value of dietary protein level effect.

4 P-value of protease supplementation effect.

a,b - Means followed by different letters differ at P<0.05 by Fisher LSD test.

Table 3 Interaction between protein and digestible amino acid levels and protease supplementation on growth performance of broilers 

Protein level Standard1 Standard 6% reduction2 6% reduction P3 SEM

Enzyme −Protease +Protease −Protease +Protease
1-7 d
BW 7 d 170 169 168 169 0.551 1.95
FI 166 162 172 165 0.845 5.75
BWG 120 120 118 120 0.530 1.98
FCR 1.37 1.35 1.45 1.38 0.617 0.03
7-21 d
BW 21 d 876 848 802 842 0.066 17.8
FI 1014 1027 1006 1030 0.771 15.1
BWG 706a 679ab 635b 673ab 0.050 16.4
FCR 1.44a 1.51a 1.58b 1.53a 0.017 0.02
21-35 d
BW 35 d 2144 2146 2040 2100 0.470 22.2
FI 2095 2094 2222 2067 0.165 32.2
BWG 1268 1298 1237 1258 0.903 19.8
FCR 1.65a 1.61a 1.80b 1.64a 0.010 0.02
35-42 d
BW 42 d 2730 2854 2664 2769 0.851 42.0
FI 1010 1194 1121 1100 0.122 55.7
BWG 568 709 624 669 0.243 29.6
FCR 1.78 1.68 1.79 1.64 0.243 0.05
1-42 d
FI 4284 4477 4521 4361 0.064 58.2
BWG 2680 2805 2615 2719 0.851 36.5
FCR 1.60a 1.60a 1.73b 1.60a 0.002 0.02

BW - body weight; FI - feed intake; BWG - body weight gain; FCR - feed conversion ratio; SEM - standard error of the mean.

1 Based on the Brazilian Tables for Poultry and Swine ( Rostagno et al., 2011 ).

2 6% reduction of crude protein and digestible amino acids.

3 P-value of interaction between protein level and protease supplementation.

a,b - Means followed by different letters differ at P<0.05 by Fisher LSD test.

To better understand the results, it is important to observe that lower indexes are indicators of better intestinal health ( Table 4 ). Birds fed diet supplemented with protease had the best results for intestinal health index. (P<0.05). This overall result was mainly due to the effect on lamina propria, epithelial thickness, and proliferation of enterocytes. The reduction on dietary protein negatively (P<0.05) affected epithelial thickness and epithelial plasma infiltration but promoted better (P<0.05) index for lamina propria thickness.

Table 4 Intestinal health evaluated by the ISI methodology of broilers fed diets containing different protein and digestible amino acid levels supplemented or not with protease 

Protein level Protease supplementation


Standard diet1 6% reduction2 SEM P3 −Protease +Protease SEM P4
Lamina propria thickness 0.86b 0.60a 0.09 0.05 0.89b 0.58a 0.09 0.02
Epithelial thickness 0.06a 0.14b 0.03 0.04 0.14b 0.05a 0.03 0.03
Proliferation of enterocytes 0.09 0.17 0.03 0.10 0.18b 0.08a 0.03 0.02
Epithelial plasma infiltration 0.65a 0.79b 0.05 0.03 0.72 0.71 0.05 0.80
Mixed inflammatory infiltration in the lamina propria 1.67 1.73 0.16 0.81 1.78 1.62 0.16 0.54
Increase of goblet cells 1.79 1.83 0.12 0.79 1.88 1.74 0.12 0.50
Congestion 0.50 0.32 0.09 0.14 0.52 0.31 0.09 0.08
Necrosis/apical karyolysis 0.13 0.05 0.05 0.28 0.07 0.12 0.05 0.56
Presence of oocysts 0.00 0.00 0.00 - 0.00 0.00 0.00 -
Total 5.74 5.63 0.26 0.72 6.18b 5.20a 0.27 0.01

SEM - standard error of the mean.

1 Based on the Brazilian Tables for Poultry and Swine ( Rostagno et al., 2011 ).

2 6% reduction of crude protein and digestible amino acids.

3 P-value of dietary protein level effect.

4 P-value of protease supplementation effect.

a,b - Means followed by different letters differ at P<0.05 by Fisher LSD test.

The lamina propria thickness had the best index with Low CP&AA+P treatment, and the epithelial thickness had the worst result with Low CP&AA−P (P<0.05; Table 5 ). The treatments SD+P and Low CP&AA+P presented better index of enterocyte proliferation, while the worst result was observed for the Low CP&AA−P treatment (P<0.05). In the sum of the health index of the ileum, we observed that the treatments with protease obtained the best results, and the SD−P diet had a numerally (P<0.06) worst ileum health index.

Table 5 Interaction between protein and digestible amino acid levels and protease supplementation on ileal digestibility coefficients of crude protein and amino acids on intestinal health evaluated by the ISI methodology 

Protein level Standard diet1 6% reduction2 SEM P3


Enzyme −Protease +Protease −Protease +Protease
Lamina propria thickness 0.96b 0.76b 0.82b 0.37a 0.09 0.02
Epithelial thickness 0.07a 0.04a 0.21b 0.07a 0.03 0.02
Proliferation of enterocytes 0.13ab 0.06a 0.23b 0.10a 0.03 0.04
Epithelial plasma infiltration 0.64 0.66 0.82 0.76 0.05 0.16
Mixed inflammatory infiltration in the lamina propria 1.91 1.44 1.62 1.83 0.16 0.39
Increase of goblet cells 2.00 1.58 1.74 1.93 0.12 0.26
Congestion 0.58 0.42 0.46 0.17 0.09 0.15
Necrosis/apical karyolysis 0.09 0.17 0.05 0.05 0.05 0.60
Presence of oocysts 0.00 0.00 0.00 0.00 0.00 -
Total 6.38 5.13 5.95 5.29 0.26 0.06

SEM - standard error of the mean.

1 Based on the Brazilian Tables for Poultry and Swine ( Rostagno et al., 2011 ).

2 6% reduction of crude protein and digestible amino acids.

3 P-value of interaction between protein level and protease supplementation.

a,b - Means followed by different letters differ at P<0.05 by Fisher LSD test

Discussion

The results of growth performance of broiler fed diets with the addition of exogenous proteases is variable, possibly due to differences in laboratory assays and experimental designs, especially in negative control diets ( Rutherfurd et al., 2007 ; Angel et al., 2011 ; Romero et al., 2014 ). The effects of protein and amino acid digestibility on growth may be associated to physical and chemical factors of the individual feed ingredients, as granule size and anti-nutritional factors ( Romero et al., 2013 ; Amha et al., 2015 ). El-Katcha et al. (2014) observed that performance index of broilers was not significantly affected by enzyme supplementation in comparison with the control group. In our trial, there was no difference in FCR between SD−P and SD+P treatments and this may be associated with optimal levels of endogenous enzymes, that is, the amount of protease present in the gut was enough to digest the protein. Another hypothesis is that the result may be associated with the adaptation of the animal. Some studies have shown that broilers reduce the endogenous secretion of enzymes when exogenous addition occurs. According to Yuan et al. (2017) , broilers fed diet supplemented with 160 and 80 mg kg−1 of protease reduced cholecystokinin and pancreatic trypsin activity.

In this study, an extreme reduction (6%) of dietary protein and digestible amino acids negatively affected FCR, although, when protease was added, the growth performance results were similar to the standard diets. Rehman et al. (2017) also observed a decrease in body weight of broilers fed diets with protein reduction, and it was associated to the low dietary protein and amino acid concentrations. These authors obtained better results of performance, feed utilization, carcass traits, and nitrogen retention when there was reduction of protein with protease supplementation. According to Cowieson and Roos (2013) , when the inherent digestibility of amino acids in the control diet was less than 70%, protease addition improved amino acid digestibility in 90% of cases with a mean improvement of around 10%. When the inherent digestibility of amino acids in the control diet was more than 90%, there was a protease-mediated improvement in digestibility in only 60% of cases with a mean improvement of around 2%. Ghazi et al. (2002) reported that the addition of protease in broiler diet resulted in increased true metabolizable energy, and Yu et al. (2007) observed that supplementation of protease in diets with low CP can reduce protein waste and nitrogen excretion, without a reduction in broiler performance. Together, these factors may explain why protease improved feed conversion in chickens fed diets with reduced protein.

The Low CP&AA−P diet had poorer BWG and FCR compared with SD−P during the starter phase (7−21 d), which is a period of increased protein requirements due to muscle deposition. Previous studies reported that growth performance was markedly affected by changes of dietary protein levels, accompanied by varying gastrointestinal digestive enzymes ( Liu et al., 2017 ; Yuan et al., 2017) . Wang et al. (2018) observed that protein level has a significant effect on trypsin and chymotrypsin activities in ducks. The addition of the protease (Low CP&AA+P) compensated the performance losses, resulting in FCR equal to the SD+P diet. Based on values of protein digestibility reported in the literature, it can be inferred that a considerable amount of protein passes through the gastrointestinal tract without being completely digested ( Lemme et al., 2004 ; Mahmood et al., 2017) . The use of exogenous proteases can enhance endogenous peptidases, improving digestibility of dietary protein and hydrolyzing some anti-nutritional factors, such as antigenic proteins and trypsin inhibitors ( Douglas et al., 2000 ; Ghazi et al., 2002 ; Yu et al., 2007 ; Dosković et al., 2013) .

Sigolo et al. (2017) reduced 2.5% in CP and broiler growth performance was negatively affected, but when threonine was included at 110% of Ross recommendations, in low protein diet, weight gain and FI were increased. Thus, it is possible that the protease added to the Low CP&AA diet increased threonine availability, consequently, improving feed conversion. Jahanian (2010) reported that threonine supplementation of low-CP diets enhances intestinal health with positive effects on performance. Threonine absorbed is destined to intestinal protein synthesis, which are secreted into the lumen as mucus. Mucins are glycosylated proteins secreted in the intestinal epithelium and are involved in the diffusion and absorption of nutrients along the digestive tract, and mucin also protects the gut from anti-nutritional factors ( Schaart et al., 2005 ; Kim et al., 2007) .

The improvement in FCR with the protease addition may be associated with the effect of the enzyme on the improvement in intestinal health of the ileum. Stressors in the digesta can alter the intestinal mucosa, and it is important to pay attention to changes that occur in the gut. Underneath the mucosa, there is a vast surface of epithelial cells of the absorptive type, essential for the transport of nutrients into the enterocytes ( Choct, 2009 ). There are indications that the greater thickness of lamina propria and mucosa is linked to the action of stressors ( Silva et al., 2009 ; Miles et al., 2006 ). By reducing the lamina propria thickness and the epithelial surface, nutrients can be transported more easily, improving absorption and helping to maintain intestinal health.

Many factors can affect the microbiota and morphology of the broiler gut, such as diet, management, and challenges ( Stanley et al., 2014 ; Cowieson et al., 2017 ). Beneficial effects of protease are associated to the amount of undigested nutrients in the ileum, such as protein and starch ( Romero et al., 2011 ; Romero and Plumstead, 2010 ). Some diet ingredients can produce microbial toxic components, negatively affecting growth performance and nutrient utilization ( Rehman et al., 2007 ). Increased indigestible protein in the hindgut can result in proteolytic fermentation in the ceca of broilers ( Wilkie et al., 2005 ). Depending on the extent of the putrefaction and amino acid composition of the fermented protein, there is a range of potentially harmful consequences ( Windey et al., 2012 ). The metabolic fates of amino acids in enterocytes of the gut mucosa can be manipulated by dietary strategies and suggest that higher concentrations of amino acids in the portal circulation are generated by supplemental rather than protein-bound amino acids ( Moss et al., 2018 ). The use of exogenous protease in this study may be associated with increased availability of protein-bound amino acids. The mechanism that explains how protease acts in intestinal health is unclear. The hydrolysis of antinutritional factors and antigenic proteins are examples of several factors that may influence intestinal health ( Ghazi et al., 2002 ; Cowieson and Roos, 2016 ). Besides, the intestinal health may be influenced by the reduction of intestinal lumen viscosity ( Odetallah et al., 2003 ), increment of AA availability for mucin synthesis ( Cowieson and Roos, 2013 ), microbiota changes ( Windey et al., 2012 ), and tight junction integrity ( Cowieson et al., 2017 ).

The tendency for increased FI of broilers in the Low CP&AA−P diet in the overall rearing period may be associated with the aminostatic theory of intake regulation ( Gonzales, 2002 ), in which broilers eat to meet their protein/amino acid requirements first. However, with protease supplementation, FI was decreased, suggesting an improvement in the balance of circulating amino acids. This difference in FI resulted in improved FCR for the protease-supplemented birds. Broilers fed the Low CP&AA diet had the poorest FCR compared with those fed the other treatments in this study, demonstrating that a 6% reduction of CP and amino acids affects growth performance of broilers, but when a protease is supplemented, growth performance may be recovered.

Conclusions

The use of an exogenous protease can improve feed conversion of broilers. The supplementation of protease positively changes lamina propria, epithelial thickness, and proliferation of enterocytes, resulting in a better intestinal health index.

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Recebido: 25 de Outubro de 2018; Aceito: 30 de Abril de 2019

* Corresponding author: katia.zootecnia@hotmail.com

Conflict of Interest

The authors declare no conflict of interest.

Author Contributions

Conceptualization: K.M. Cardinal, M.L. Moraes, I. Andretta, E. Santin and A.M.L. Ribeiro. Data curation: K.M. Cardinal, M.L. Moraes, I. Andretta, G.D. Schirmann, E. Santin and A.M.L. Ribeiro. Formal analysis: K.M. Cardinal, I. Andretta, E. Santin and A.M.L. Ribeiro. Investigation: K.M. Cardinal, M.L. Moraes, I. Andretta and A.M.L. Ribeiro. Methodology: K.M. Cardinal, M.L. Moraes, I. Andretta, B.L. Belote, M.A. Barrios, E. Santin and A.M.L. Ribeiro. Project administration: K.M. Cardinal, I. Andretta and A.M.L. Ribeiro. Supervision: K.M. Cardinal, I. Andretta, B.L. Belote, M.A. Barrios and A.M.L. Ribeiro. Validation: K.M. Cardinal, M.L. Moraes, I. Andretta, B.L. Belote, E. Santin and A.M.L. Ribeiro. Visualization: K.M. Cardinal, M.L. Moraes, I. Andretta and A.M.L. Ribeiro. Writing-original draft: K.M. Cardinal, M.L. Moraes, I. Andretta and A.M.L. Ribeiro. Writing-review & editing: K.M. Cardinal, I. Andretta and A.M.L. Ribeiro.

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